Modern heating pads and electronic blankets have heater wires (conductors) that do not require separate thermostats. They fall into two basic types: a heater wire having a positive temperature coefficient (PTC) heating layer arranged between two conductors that exhibits an increased resistance with an increase in temperature so that the wire is self-limiting and not subject to hot spots; and a heater wire that provides a feedback signal to a control for monitoring temperature and detecting local hot spots.
A prior art system that uses a feedback signal for temperature control concurrently with a voltage, that also indicates the occurrence of a hot spot that deteriorates the insulation between a heater conductor and a sensor or sensor wire, is described in U.S. Pat. No. 5,861,610. A PTC nickel alloy sensor wire is counter-wound around a heater wire with an inner insulation therebetween. Current leakage through the insulation electrically couples the sensor wire and the heater wire. Resistance of the sensor wire is measured and used for temperature control. An alternating current (AC) voltage present on the sensor wire indicates the existence of a breakdown in the separating insulation. When polyvinylchloride (PVC) is used as the separating layer, small leakage occurs at about 160° C. When polyethylene is used as the separating layer, the layer melts at about 130° C. and contact is made between the heater wire and the sensor wire. In both cases, i.e., when leakage occurs or contact between the heater wire and the sensor wire is made, the control unit disconnects power to the heater wire.
A similar technique is disclosed in U.S. Pat. No. 6,310,332 (Gerrard), the entire disclosure of which is incorporated herein by reference, wherein a second conductor is used as a heater with the insulation having an enhanced Negative Temperature Coefficient (NTC) characteristic. The two heating conductors are connected through a diode so that leakage through the NTC layer introduces the negative half cycle, which presence causes termination of the power. In a second embodiment, the second conductor is a PTC sensor wire, such as disclosed in U.S. Pat. No. 5,861,610, the entire disclosure of which is incorporated herein by reference.
A smaller more flexible heater wire design is disclosed in U.S. Pat. No. 6,222,162 (Keane), the entire disclosure of which is incorporated herein by reference, and uses a single conductor of a PTC alloy for both heating and temperature sensing. In this device, only the average temperature is used to control the temperature of the wire.
To address these concerns, a heater wire is disclosed in U.S. Pat. No. 7,180,037 (Weiss), the entire disclosure of which is incorporated herein by reference, which is operated with an alternating current power supply. The heater wire has a conductive core with an NTC layer and a helically wound sensor conductor within an insulative outer sheath. The conductive core is coupled to a control circuit, with a phase shift relative to the AC power supply being indicative of the temperature of the wire.